1. Field of the Invention
The invention relates generally to alternator systems for charging secondary batteries and particularly to an alternator system allowing manual selection of charging current and having the capability of discontinuing charging current on attainment of full charge or in the event of conditions capable of damaging a battery.
2. Description of the Prior Art
Secondary batteries are commonly used in situations where electricity is drawn from the battery during a period of time when recharging power is not available, electricity thus being drawn into the battery during one time interval with recharging occurring during another time interval. Particular situations where these conditions exist occur on boats, recreational vehicles and other situations wherein electricity is needed for function such as lighting, refrigeration, etc. during periods when the battery is not being recharged through operation of an engine. In such situations, a battery must be recharged during subsequent engine operation and at a charging rate which is commensurate with a reasonable period of engine operation. In practice, recharging of such batteries with high currents during a relatively short recharging time is not possible due to deleterious effects within the battery of heat generation and chemical reactions occurring at battery plates which would be caused by recharging at too high a current rate and within a period of time which is too short for a given battery. In the prior art, the risk of overheating a battery by a high charging current exists where relatively small batteries are connected to sources of charging current with potentially high output capacity. In such situations, alternator systems must be operated in concert with a regulator which automatically limits battery charging current. Prior regulators used for limiting the battery charging current typically limit the applied voltage although other variables can be employed. The recharging of lead-acid batteries, the most common battery used on boats, recreational vehicles and in other similar situations, is usually made automatic by the maintenance of a preselected voltage which is applied to the battery. Battery charging by regulation of voltage invariably relies on the gradual increase of battery voltage to cause charging current to diminish or stop when the battery reaches full charge. A primary disadvantage of these prior systems in the length of time needed to recharge the last third to one-half of a battery's capacity. Even though recharging to approximately half of the battery's capacity occurs relatively quickly, complete recharging requires a much greater period of time due to the fact that recharging becomes increasingly ineffective.
Prior systems employed to charge secondary batteries are exemplified by U.S. Pat. No. 4,136,311 to Scheidler who describes the use of a high battery charging current during initial charging followed by voltage switching to a lower rate for completion of charge. Watrous, in U.S. Pat. No. 4,286,205, provides an "add-on" circuit which is additional to an alternator and voltage regulator, the additional circuit supplying an added field current so that the alternator can be used to supply higher than normal power output. Chait, in U.S. Pat. No. 3,193,750, uses battery voltage to control an SCR and uses a zener diode to facilitate use of a high battery charging current until battery voltage reaches a predetermined point, whereupon charge rate is reduced to a "trickle" condition. In U.S. Pat. No. 4,315,205, Mori describes apparatus capable of switching from a normal voltage regulation for battery charging to a lower voltage regulation in case of failure in portions of the charging system. Burkett, in U.S. Pat. No. 3,436,639 avoids battery overcharging by providing a voltage sensitive trigger circuit having at least one zener diode and voltage divider resistance for triggering an SCR, the circuit shorting a generator or alternator and parallel to an alternator and battery, the circuit bypassing excess charging current to ground.
The prior art, including those systems exemplified by the patents referred to above, typically function by sensing the existence of high voltage conditions, acting to reduce the potential for damage occasioned by the high voltage condition, and then returning to a first set of conditions wherein the apparatus continues to function. In order to provide desirably safe operating conditions, there is a need in the art for an alternator system having a high recharging rate wherein charging current can be manually set according to a set of factors including estimated engine operating time and whereby operation of the alternator can be discontinued in the event of conditions potentially damaging to the battery.